1. Field of the Invention
The present general inventive concept relates to an apparatus and method of suppressing artifacts of an image signal and a method thereof, and more particularly to an apparatus to suppress artifacts generated in an image signal and a method thereof.
2. Description of the Related Art
Human eyes perceive a continuous image in a moving picture when viewing 16 or more frames per second. In a moving picture, 16 frames per second correspond to a minimum sampling frequency (i.e., Nyquist frequency) for sampling a signal with the signal's information preserved. In consideration of this, a movie image is processed at a speed of 24 frames per second, and a TV image is processed at a speed of 25 to 30 frames per second.
Movies typically use a progressive system that instantaneously stores every picture in a film and progressively projects the pictures onto a screen. With television, since an image is basically transmitted over the air, each picture is filmed and transmitted through scanning of several hundreds of scanning lines, and then is displayed on a screen of a Braun tube by the scanning. In the NTSC (National Television System Committee) color TV system adopted in the United States, Japan, Korea, etc., 30 frames, each of which is composed of 525 scanning lines per second, are transmitted. In the PAL (Phase Alternation by Line) system or SECAM (Sequential Couleur a Memoire) system, 25 frames, each of which is composed of 625 scanning lines per second are transmitted.
Generally, TV uses an interlaced scanning method that divides one picture (i.e., frame) into two fields and alternately scans the two fields in order to effectively present a moving image using limited scanning lines. The divided fields are called top and bottom fields, odd and even fields, upper and lower fields, etc. Accordingly, the NTSC system processes 60 frames per second, and the PAL or SECAM system processes 50 frames per second.
When a movie is televised through a TV, every frame of the movie is transmitted through a converter called a telecine (which is a compound word of a television and a cinema). Currently, if the movie is reproduced through the TV without matching the number of pictures with that of TV, the movie is displayed on the TV screen at a higher frame rate than a normal frame rate. In the case of transmitting the movie through the NTSC TV system, 60 fields should be obtained from 24 pictures (i.e., frames) per second. Thus, two fields should be obtained from 2 pictures, in order to match the picture speed. A simple and widely used method is called a “3:2 pull-down” system, which scans three fields with respect to one picture, and scans two fields with respect to the other picture.
In principle, a DVD (Digital Video Disc) can provide an image of 24 frames in the same manner as the original movie. However, since most display appliances, such as TVs, adopt the interlaced scanning system, the DVD is actually made to adopt the interlaced scanning system. However, a personal computer or a digital TV adopts the progressive scanning system, and thus a de-interlacing operation, which is the reverse of the 3:2 pull-down operation, should be performed to restore the image frames of the interlaced scanning system to the progressive scanning system.
Three de-interlacing methods for converting the image signal of the interlaced scanning system into the image signal of the progressive scanning system have been used.
A first method of de-interlacing is to remove one of the two fields. This method provides a frame by removing one field and blending or interpolating the other field. This method has the drawbacks in that the resolution of the image deteriorates by half and a specified image at a specified time point may disappear due to the removal of one field.
A second method of de-interlacing is called a blending or weaving method. This method provides a field by combining a top field and a bottom field. According to this method, one frame is provided by simply inserting a line of the previous field between lines of the present field. In the case of interpolating a still image, this method can be easily implemented. However, in the case of interpolating a moving image, horizontal lines may appear on a displayed picture or the displayed picture may deteriorate. That is, when there is no temporal gap between the fields of the same frame of a telecine image, this method can provide a perfect progressive image through proper processing of the image. However, since there is a temporal gap of about 1/60 of a second between the fields of the different frames, image inconsistency exists between the fields of the images in motion, and this causes the picture quality to deteriorate when the image is blended.
A Third method of de-interlacing is called a BOB or line doubling method. This method provides a frame by using the respective lines of a field twice. In other words, a new frame is provided by inserting average data of two lines into an area between the two lines in a field. This inter-field interpolating method prevents the horizontal lines from occurring in a displayed image in motion. However, in the case of interpolating a still image, the displayed picture deteriorates, and a complicated and minute picture flickers with 30 Hz.
As described above, some de-interlaced artifacts may be generated in the de-interlaced image signal, and such de-interlaced artifacts become greater as the picture frames are switched or the image moves fast. Accordingly, a post-process for suppressing the de-interlaced artifacts and obtaining an image of a high picture quality is required.
FIG. 1 is a block diagram schematically illustrating a conventional apparatus for suppressing de-interlaced artifacts. Referring to FIG. 1, the de-interlaced artifact suppressing apparatus includes a converter 10, a sawtooth artifact detector 20 and a vertical averager 30.
The converter 10 converts an image signal of an interlaced scanning system into an image signal of a progressive scanning system. The conversion of the image signal of the interlaced scanning system into the image signal of the progressive scanning system is performed using any one of the three methods as described above. Also, other conversion methods can be used as well.
The sawtooth artifact detector 20 determines an area in which sawtooth artifacts are produced if the area has a greater pixel difference value than a threshold value between two adjacent horizontal scanning lines of the de-interlaced image signal, and has a smaller pixel difference value than the threshold value between two scanning lines skipping over one scanning line. Here, the sawtooth artifact is a phenomenon where a difference between a scanning line, which is interpolated through a de-interlacing process and located between two adjacent horizontal scanning lines of the interlaced image signal having similar properties, and the scanning line of the interlaced image signal.
The condition that a pattern of sawtooth artifacts is detected through the sawtooth artifact detector 20 is in equation 1 and equation 2 as follows:if |line n−line (n+2)|≅0 and |line n−line (n+1)|< >0  Equation 1orif |line n−line (n−1)|≅|line n−line (n+1)|< >0  Equation 2
That is, the sawtooth artifact detector 20 decides that the pattern of the interpolated scanning lines is the pattern of the sawtooth artifacts if the area has a greater pixel difference value than a threshold value between two adjacent horizontal scanning lines of the de-interlaced image signal and has a smaller pixel difference value than the threshold value between two scanning lines skipping over one scanning line, or if the difference between the interpolated scanning line and the adjacent horizontal scanning lines is greater than the threshold value.
The vertical averager 30 removes the sawtooth artifacts by filtering the area determined to be the sawtooth artifact area in a vertical direction. At this time, a 5-pole vertical-direction filter is used in order to remove the sawtooth artifacts. The 5-pole vertical-direction filter has a rise time suitable in a pre-shoot or an over-shoot, is low-priced, and has a low complexity.
FIG. 2 is a view illustrating an image signal in which sawtooth artifacts are suppressed by the sawtooth artifact suppressing apparatus of FIG. 1. In FIG. 2, a horizontal axis represents positions of scanning lines in a vertical direction, and a vertical axis represents pixel values according to the positions of the scanning lines in the vertical direction. Also, the solid line represents an image signal before the sawtooth artifacts are suppressed, and the dotted line represents an image signal after the sawtooth artifacts are suppressed.
However, the conventional sawtooth artifact suppressing apparatus, which detects an area in which the sawtooth artifacts are generated by comparing the difference between the scanning lines with a threshold value, has the drawbacks in that the generated sawtooth artifacts may be lost due to the set threshold value. Another drawback is that an area in which no sawtooth artifacts are generated may be misrecognized as the sawtooth artifact area.
Also, in the case of an image with a heavy motion, the de-interlaced artifacts become distorted, and this causes the sawtooth artifacts to also be distorted. In this case, no efficient suppression of the sawtooth artifacts can be obtained through just the vertical filtering of the image signal.